The invention relates to encoder devices and more particularly to devices for providing signal outputs representing the position of a printwheel.
Printwheel encoders are well-known and are described, for example, in U.S. Pat. No. 3,978,457 and in 4,313,105 in connection with postage meter printwheels. Because of the great need for security in devices such as postage meters which in effect are printing money, many of the known encoder devices which provides mechanical switching contact rising wipers are normally not sufficiently rugged to reach the number of cycles anticipated for the meter. In addition, the environment of postage meters, involving as it does paper dust and envelope glue and water, militates against such mechanical devices because of the expense of the necessary protection against the environmental factors.
In order to avoid this aspect of the environmental problems and to achieve greater life expectancy, optical encoders have been used in various ways in postage meters. While optical encoders work well, there are many cases in which the requirement for encoding necessitates an increased volume of the setting mechanism for the printwheels simply because of the extra space necessary to accommodate the encoder and an encoding disc.
Hall-effect sensors have also been used in postage meters. Magnets have been mounted on the setting rack and the magnetic position is sensed to aid in determining the position of the rack driving a printwheel as shown in U.S. Pat. No. 4,398,458. Magneto-restrictive sensors are shown in U.S. Pat. No. 4,224,603. This last cited patent teaches apparatus which requires that the print drum and its racks be in motion in order to determine the setting of the printwheels.
Hall-effect sensors have also been used to generate position-dependent pulses for synchronization. Such an application is shown in U.S. Pat. No. 3,939,372 in which a flux-conducting cam is brought near the sensor as a shaft rotates in order to generate a signal pulse from the sensor. Also suggested therein is a spiral shaped magnet which in conjunction with the Hall-effect device may be used to provide position information.
In many setting applications and particularly in postage meter printwheel setting mechanisms, there is a need to provide encoding in a small volume and preferably at much less cost than the present optical encoders.
It is therefore an object of the invention to provide an encoder which solves the problem of encoding in a relatively dirty environment and in a small volume.
It is another object to provide an absolute position encoding apparatus particularly for printwheels.
It is yet another object to provide an absolute position encoder for a printwheel which provides security against environmental disturbances.